This invention relates generally to metal founding, and more particularly to an improved method of and apparatus for casting molten metal in a continuous casting machine of the wheel-belt type.
In known continuous casting systems, molten metal is flowed into a mold formed by enclosing an arcuate portion of the peripheral groove of a rotating casting wheel with a flexible endless metal belt. As the casting wheel rotates, a coolant is applied to the external surfaces of the wheel adjacent the peripheral groove and to the flexible belt to prevent excessive heating thereof and to achieve rapid solidification of the molten metal. As the cast metal travels between the rotating casting wheel and the circumferential belt, it solidifies and emerges from between the two surfaces as a continuous bar, ready to be rolled or otherwise worked. For the purposes of illustration, the present invention will be described with specific reference to its use in a continuous copper bar or a continuous aluminum bar casting operation such as just mentioned, but it is to be understood that the invention in its broader aspects may be utilized with other metals and other continuous casting processes that employ moveable casting surfaces.
When continuously casting metals in machines of the above described type, it is highly advantageous to solidify the molten metal in as short a period of time as possible in order to maintain a high casting rate. Moreover, when casting metals containing alloying elements, rapid solidification is desired in order to maintain intermettalic compounds in solid solution and to limit the size of the particles that precipitate out of solution. However, the low cooling efficiency and non-uniformity of heat transfer inherent in the thick metal belts (0.060 inches or greater) of prior art casting machines preclude attainment of the high casting rates and metallurgical effects desired. This is especially so if the belt is fabricated of a material having a relatively low rate of heat transfer even though coolant is applied to the external belt surface in maximum practical pressure and volume.
It should be apparent, therefore, that to improve the casting rate of continuous casting machines of the type described concommitantly with the life expectancy of the flexible belt element, belts so formed must have a high heat transfer rate to achieve such improvements. However, the prior art recognizes several problems in attempting to fabricate the belt from thin gauge and/or high heat transfer materials. Notably, one problem encountered when fabricating mold components of high heat transfer rate materials is the typically low structural strength of such materials which adversely affects their useful life, as explained in U.S. Pat. No. 3,464,483. While reducing belt thickness would tend to reduce belt strain exerted by the belt wheels on a belt formed of a high heat transfer rate material, the structural strength of the belt would, of course, be further diminished so that little advantage in terms of belt life could be realized.
A further problem associated with the use of thin gauge belt elements for continuous casting machines is explained in U.S. Pat. No. 3,533,463, wherein the patentees recognize the susceptibility of damage to a thin gauge belt, particularly at the edges thereof, when it is adequately tensioned to prevent leakage of molten metal from between the belt and casting wheel groove. It is further noted in the aforesaid patent that the belt wheel flanges used to steer the belt into position for enclosing the groove are especially damaging to the edges of a thin gauge belt.
One prior art method and apparatus for improving belt life and casting rate is described in U.S. Pat. No. 3,642,055, wherein a foraminous wire mesh belt is employed to close the peripheral groove of the casting wheel of a wheel-belt type continuous casting machine. Coolant is directed through the openings in the belt to impinge directly upon the molten metal in the groove. The high surface tension of the molten metal is relied upon to prevent the molten metal from flowing through the openings in the foraminous belt. However, at the high casting rates contemplated by the present invention, the surface tension of the molten metal would very likely be insufficient to prevent leakage through the foraminous belt with the resultant danger of metal splatter in the area surrounding the casting machine. Even if leakage could be prevented, there always exists the possibility of obstructing the pores of the foraminous belt with solidified metal, rendering the escape of the vaporized coolant from the casting groove more difficult and thus increasing the danger of explosion. Moreover, the separation of the cast bar from the foraminous belt as the bar exits the casting groove could be hampered because of adherence between the cast bar and foraminous belt as the molten metal in contact with the belt solidifies. A further disadvantage of the apparatus and method described in U.S. Pat. No. 3,642,055 is the resulting poor quality of the cast bar which confronts the coarse and irregular surface of the foraminous belt.